Service manual

CB-19
Video Processing
AKB signal is derived from the collectors of Q5124, 5144, and 5164. These are fed back to the AKB
circuit via pin 5 of the 5C2 connector and processed in the manner described above.
The CRT board also features a setup jumper for optimal G2 adjustment. The purpose for it is to force DC
voltage on the cathodes to be equal to the CRT cut-off voltage (170-190 volts DC with respect to G1).
When jumped, the G2 setup circuitry grounds any incoming video signal. The servicer can then adjust the
G2 pot, located on the flyback transformer, TX3204, until retrace lines just start to appear on the screen.
Going back to the example of the green circuit, current then flows in Q5141, D5142, L5141, and RX5146.
It is at this point that raster is almost cut off. When the jumper is released, the AKB circuit adjusts the color
with the highest CRT cut-off voltage to the G2 setup voltage. The other two colors will be just slightly lower
depending on their cut-off settings in the service menu.
1
12
11
10
9
8
7
6
5
4
CRT1
1
2
3
4
1 2 3 4 5 6 7
+235V
Q5121
Q5123
Q5101
Q5124
1
2
D5122
D5121
RX5129
PROV
RX5128
R5173
Jumper
L5121
RX5126
RX5127
L5122
RX5130
Q5122
R5122M
R5121M
Q5141
Q5143
Q5144
D5142
D5141
RX5149
PROV
RX5148
R5172
Jumper
L5141
RX5146
RX5147
L5142
RX5150
Q5142
R5142
C5143
PROV
R5141M
Q5161
Q5163
Q5164
D5162
D5161
RX5169
PROV
RX5168
R5171
Jumper
L5161
RX5166
RX5167
L5162
RX5170
Q5162
R5162M
C5163
PROV
R5161M
C5123
PROV
+12VSW
R5165
R5164M
R5163M
C5161M
R5145M
R5144M
R5143M
C5141M
+12VSW
+12VSW
R5125M
R5124M
R5123M
C5121M
+12VSW
C5105
R5107
C5104
PROV
R5110
PROV
+235V
C5100
W5101
R5101
PROV
R5106
C5103
G2
W5102
PROV
DAG1
DAG2
C5106
D5101
R5109
R5104
R5103
R5102
R5105
+235V
C5102
PROV
C5101
L5101
C5164
PROV
5C2 KEY
1: Blue
2: Green
3: Red
4: GND
5: AKB_Sense
6: Setup
7: 12VSWDGND
SETUP
BLUE
GREEN
RED
5F2
5C2
<<Figure CB-17>>
The afterglow suppression circuit is used to prevent any residue light that might otherwise remain on the
screen after shutdown. This circuit takes advantage of the fact that the filter capacitor on the +235V line
takes a relatively long period of time to discharge. The loss of filament voltage at shutdown triggers the
switching of Q5101, which then briefly interrupts the G1s (pin 5) path to ground. The residue voltage on